ABSTRACT

Vellore Municipality has been upgraded with Corporation status. The

steady incremental in the city population results in the increase of domestic sewage

generation. But still now there is no treatment plant. So it is required to construct a

Sewage Treatment Plant with sufficient capacity to treat the increased sewage.

The project deals with the design of the Sewage Treatment plant and its

major components such screening chamber, grit chamber, skimming tank,

sedimentation tank, secondary clarifier, active sludge tank and sludge drying beds.

The project covers the 10.54 sq.km, 48 wards of Vellore Municipal

Corporation for next 30 years and its increased population. Vellore City, the Head

Quarters of the Vellore District is at a distance of 135 km West of Chennai and 35

km south of Chittoor Town (Andhra Pradesh).

With regard to Vellore, almost the entire town and environment are plain and

the general slope is from West to East. The town is situated at the altitude of

12°S5°N latitude and 78°E longitude. The soil of the area is being gravel, rocky

and a large proportion of sand and gravel. All the aspects of Vellore’s climate and

topography, its population growth rate is to be considered while designing the

project.

By the execution of the project the entire sewage of the city can be treated

effectively and efficiently.

1

ABBREVIATIONS

2

Qo (m3 / h) Influent flow - rate

Qe (m3 / h) Effluent flow - rate

Qr (m3 / h) Recycled sludge flow - rate

Qw (m3 / h) Wasted sludge flow - rate

BOD (mg / L) Biochemical oxygen demand

BODo (mg / L) Influent biochemical oxygen demand

SS (mg / L) Suspended solids (SS)

SSr , w (mg / L) Recycled and wasted sludge SS

A (m3 / h) Air flow - rate

MLSS (mg / L) Mixed liquor suspended solids

t (h) Hydraulic retention time

OL (kg BOD / m3 . day) Organic loading

F / M (kg BOD / kg MLSS . day)

Food to microorganism ratio

R Recycle ratio

SA (day) Sludge age

ASR (m3 / kg BOD) Air supply rate

E (%) BOD removal efficiency

Cd Co-efficient of discharge

INTRODUCTION

SEWERAGE – GENERAL CONSIDERATIONS

Sewage treatment is the process of removing contaminants from wastewater

and household sewage, both runoff (effluents) and domestic. It includes physical,

chemical, and biological processes to remove physical, chemical and biological

contaminants. Its objective is to produce a treated effluent and a solid waste or

sludge suitable for discharge or reuse back into the environment. This material is

often inadvertently contaminated with many toxic organic and inorganic

compounds.

Sewage implies the collecting of wastewaters from occupied areas and

conveying them to some point of disposal. The liquid wastes will require treatment

before they are discharged into the water body or otherwise disposed of without

endangering the public health or causing offensive conditions.

As the cities have grown, the more primitive method of excreta disposal

have gain place to the water-carried sewerage system. Even in the small cities the

greater safety of sewerage, its convenience, and freedom from nuisance have

caused it to be adopted wherever finances permit.

DEFINITIONS

Sewerage is the art of collecting, treating and finally disposing of the

sewage.

Sewage is liquid, consists of any one or a mixture of liquid waste origins

from urinals, latrines, bath rooms, kitchens of a dwelling, commercial building or

institutional buildings.

Storm sewage is a liquid flowing in sewer during or following a period of

rainfall and resulting there from.

3

A Partially Separate Sewer System is the sewerage system in which the

domestic sewage is carried with the storm water in the rain season.

Activated sludge is the active biological floc produced in activated sludge

plants, largely composed of saprotrophic bacteria, protozoan flora (amoebae) and a

range of other filter feeding species.

Mixed Liquor Suspended Solids (MLSS) is the amount of suspended solids

in the mix of raw water and activated sludge.

Return activated sludge (R.A.S) is the activated sludge extracted from the

system and mixed with raw water to form the mixed liquor.

Waste activated sludge (W.A.S.) or Surplus Activated Sludge (S.A.S.) is

excess activated sludge that is extracted from the system to be directed to sludge

treatment.

Sludge Age is the average residence time of biological solids in the system.

It can be defined as the average lifespan of bacteria in the system.

Overflow rate / Surface loading is the discharge per unit of plan area. This

parameter is the design factor in designing the settling tanks.

Food to Micro-organisms ratio (F/M ratio) is the ratio between daily BOD

load applied to Aerator System and total microbial mass in the system.

TREATMENT OF SEWAGE

4

The treatment of sewage consists of many complex functions. The degree of

treatment depends upon the characteristics of the raw inlet sewage as well as the

required effluent characteristics.

Treatment processes are often classified as:

(i) Preliminary treatment

(ii) Primary treatment

(iii) Secondary treatment

(iv) Tertiary treatment.

PRELIMINARY TREATMENT:

Preliminary treatment consists solely in separating the floating materials

like tree branches, papers, pieces of rags, wood etc. and heavy settable inorganic

solids. It helps in removal of oils and greases and reduces the BOD by 15% to

30%. The processes under this are:

➢ Screening – to remove floating papers, rags, clothes.

➢ Grit chamber – to remove grit and sand.

➢ Skimming tank – to remove oils and greases.

PRIMARY TREATMENT:

Primary treatment consists in removing large suspended organic solids. It is

usually accomplished by sedimentation in settling basins. The liquid effluent from

the primary treatment often contains a large amount of suspended organic material

and has a high BOD (about 60% of original).

SECONDARY TREATMENT:

Here the effluent from primary treatment is treated through biological

decomposition of organic matter carried out either aerobic or anaerobic conditions.5

Aerobic Biological Units:

I) Filters ( intermittent sand filters, trickling filters)

II) Activated Sludge Plant (feed of active sludge, secondary settling tank

and aeration tank)

III) Oxidation ponds and Aerated lagoons.

Anaerobic Biological Units:

I) Anaerobic lagoons

II) Septic tanks

III) Imhoff tanks.

The effluent from the secondary treatment contains a little BOD (5% to 10%

of original) and may contain several milligrams per litre of s DO.

TERTIARY TREATMENT:

The purpose of tertiary treatment is to provide a final treatment stage

to raise the effluent quality before it is discharged to the receiving environment

(sea, river, lake, ground, etc.). More than one tertiary treatment process may be

used at any treatment plant. If disinfection is practiced, it is always the final

process. It is also known as "effluent polishing".

DESIGN PERIOD:

A sewerage scheme involves the laying of underground sewer pipes and

construction of costly treatment units, which cannot be replaced or increased in

their capacities easily or conveniently at a later date. In order to avoid such 6

complications, the future expansions of the city and consequent increase in the

sewage quantity should be forecasted to serve the community satisfactorily for a

reasonable year. The future period for which the provision is made in designing the

capacities of various components of the sewerage is known as design period. This

sewage treatment plant is designed for 30 years.

PARAMETERSRAW SEWAGE OF VELLORE Corp.*

EFFLUENT

(expected)**

pH 6.4 5.5-9.0

BOD 300 mg/l ≤ 20 mg/l

COD 600 mg/l ≤ 250 mg/l

Oil & Grease 50 mg/l ≤ 5 mg/l

Total Suspended Solids 600 mg/l ≤ 30 mg/l

Nitrogen 61 mg/l ≤ 5 mg/l

Ammonia Nitrogen 50 mg/l ≤ 50 mg/l

Total Phosphorus (as PO4)

5 mg/l ≤ 5 mg/l

Total Coli form 100000 MPN/ml ≤ 1000 no/100 ml

* - Raw sewage characteristics, tested in Environmental laboratory with

Technical division, Vellore Corporation.

** - Expected effluent characteristics according the design.

POPULATION FORECAST:

Forecasting method: Incremental increase method.

Year Population Incremental Incremental increase

7

1951 1,06,0247,718

1961 1,13,742 25,430 17,622

1971 1,39,082 35,165 9,825

1981 1,74,247 814 -34,351

1991 1,75,061 2,352 1,538

2001 1,77,413

Avg =71,389 Avg = -5,366

x = 71,3895 = 14,278.

y = -53664 = -1342.

Pn = P0 + nx + n n+12 x y

Base period as 2010,

P2010 = 1,77,413 + 0.9 x 14278 + 0.9 0.9+12 x (-1342)

= 1,89,116.

Intermediate period as 2025,

P2025 = 1,77,413 + 2.4 x 14278 + 2.4 2.4+12 x (-1342)

= 2,10,343.

Ultimate design period as 2040,

8

P2040 = 1,77,413 + 3.9 x 14278 + 3.9 3.9+12 x (-1342)

= 2,45,920

At design period of 30 years the forecasted population of the Vellore city is

2,45,920.

9

CALCULATION OF SEWAGE GENERATION:

Ultimate design period = 30 years

Forecasted population at 2040 = 24.920

Per Capita Water Supply = 135 lpcd

Avg. water supply per day = 24920 x 135

= 33199200

≈ 33200000 = 33.2 MLD

Avg. sewage generation per day = 80% of supplied water

= 0.8 x 33.2

= 26.56 MLD

In cumec,

Avg. sewage generation per day = 26.56 X 1061000 X 24 X 60 X 60

Avg. discharge = 0.308 cumec

Max. discharge = 3 x avg. discharge

= 3 x 0.308

= 0.924 cumec

10

SEWAGE TREATMENT PROCESS

GENERAL

Sewage contains various types of impurities and disease

bacteria. This sewage is disposed of by dilution or on land after its collection and

conveyance. If the sewage is directly disposed of, it will be acted upon the natural

forces, which will convert it into harmful substances. The natural forces of

purification cannot purify any amount of sewage within specified time. If the

quantity of sewage is more, then receiving water will become polluted or the land

will become sewage sick. Under such circumstances it becomes essential to do

some treatment of the sewage, so that it can be accepted by the land or receiving

water without any objection. These treatment processes will directly depend on the

types of impurities present in the sewage and the standard up to which treatment is

required.

OBJECT OF TREATMENT

The main object of treatment units is to reduce the sewage contents

(solids) from the sewage and remove all the nuisance causing elements and change

the character of the sewage in such a way that it can be safely discharged in natural

water course applied on the land.

In other words, the objective of sewage treatment is to produce a

disposable effluent without causing harm or trouble to the communities and prevent

pollution.

Practically the treatment of sewage is required in big cities only where

the volume of the sewage is more as well as the quantity of various types of solid,

industrial sewage etc. is more and porous land or large quantity of water bodies is

not available for the proper disposal of sewage.

DEGREE OF TREATMENT11

The degree of treatment will mostly be decided by regulatory

agencies and the extent to which the final product of treatment are to be utilized.

The regulatory bodies might have laid down standard for the effluent or might

specify the condition under which the effluent must be discharged into the natural

stream. The method of treatment adopted should not only meet the requirement of

the regulatory bodies, but also result in the maximum use of the end product with

economy.

DESIGN PERIOD

The treatment plant is normally designed to meet the requirement

over a 30 year period after it completion. The time lag between the design and

completion should not normally exceed 2-3 years. Care should be taken that the

plant is not considerably under loaded in the initial stages, particularly the

sedimentation tank.

The ultimate design period should be 30 years and to that extent

sufficient accommodation should be provided for all the units necessary to cater to

the need of ultimate population. In some cases, it may be necessary to combine a

number of sewage systems with a common sewage treatment plant.

LOCATION OF TREATMENT PLANT

The treatment plant should be located as near to the point of disposal

as possible. If the sewage as to be disposed finally in to the river, the plant should

be located near the river bank. Care should be taken while locating the site that it

should be on the downstream side of the city and sufficiently away from water

intake works. If finally the sewage as to be applied on land, the treatment plant

should be located near the land at such a place from where the treated sewage can

directly flow under gravitational forces toward the disposal point. The plant should

not be much far away from the town to reduce the length of the sewer line.

12

On the other hand the site should not be close to the town, that it

may cause difficulties in the expansion of town and may pollute the general

atmosphere by smell and fly nuisance.

LAYOUT OF TREATMENT PLANT

The following point should be kept in mind while giving layout of

any sewage treatment plant:

• All the plant should be located in the order of sequence, so that

sewage from one process should directly go to other process.

• If possible all the plant should be located at such elevation that sewage

can flow from one plant into next under its force of gravity only.

• All the treatment units should be arranged in such a way that

minimum area is required it will also ensure economy in its cost.

• Sufficient area should be occupied for future extension.

• Staff quarter and office also should be provided near the treatment

plant, so that operators can watch the plant easily.

• The site of treatment plant should be very neat and give very good

appearance.

• Bypass and overflow weir should be provided to cut out of operation

any unit when required.

All channels, conduits should be laid in such a way as to obtain flexibility,

convenience and economy in the operation.

13

14

15

POINT CONSIDERED IN DESIGN:

Following points are considered during the design of sewage treatment unit:

• The design period should be taken between 25 to 30 years.

• The design should not be done on the hourly sewage flow basis, but

the average domestic flow plus the maximum industrial flow on the yearly record

basis.

• Instead of providing one big unit for each treatment more than two

numbers small units should provided, which will provide in operation as well as no

stoppage during maintenance and repair of the plant.

• Overflow weirs and the bypasses should be provided to cut the

particular operation if desired.

• Self cleaning velocity should develop at every place and stage.

• The design of the treatment units should be economical; easy in

maintenance should offer flexibility in operation.

16

RECEIVING CHAMBER

Receiving chamber is the structure to receive the raw sewage collected through Under Ground Sewage System from the city. It is a rectangular shape tank constructed at the entrance of the sewage treatment plant. The main sewer pipe is directly connected with this tank.

DESIGN:

Design flow = 0.924 cumec

Detention time = 60 sec

Volume required = flow X detention time

= 0.924 x 60

Vrqd = 55.44 m3

Provide, depth = 3m

Area = 55.443

= 18.48 m2

Length: Breadth = 2:1

L x B = 2B x B =2B2 = 18.48

B = 3m

L = 6.2m

CHECK:

Volume designed = 6.2 x 3 x 3

Vdes = 55.8 m2

Vrqd = 55.44 m3

Vdes > Vrqd

Receiving chamber is designed for the size of

6.2m X 3m X 3m (SWD) + 0.5 (FB)

17

18

SCREENING

GENERAL:

Screening is the very first operation carried out at a sewage treatment

plant and consists of passing the raw sewage through different types of screens so

as to trap and remove the floating matter such as tree leaves, paper, gravel, timber

pieces, rags, fibre, tampons, cans, and kitchen refuse etc.

PURPOSE OF SCREENING:

Screening is essential in sewage treatment for removal of materials

which would otherwise damage the plant, interfere with the satisfactory operation

of treatment unit or equipment.

• To protect the pumps and other equipments from the possible damages

due to floating matter.

• To remove the major floating matters from the raw sewage in a simple

manner before it reaches into the complex high energy required

process.

COARSE SCREENS

` The coarse screens essentially consist of steel bars or flat placed 30°

to 60° inclination to the horizontal. The opening between bars are 50mm or above.

These racks are placed in the screen chamber provided in the way of sewer line.

The width of the rack channel should be sufficient so that self cleaning

velocity should be available and a bypass channel should be provided to prevent

the overtopping. The bypass channel is provided with vertical bar screen. A well

drained trough is provided to store the impurities while cleaning the rack. These

racks are cleaned mechanically.

19

DESIGN OF COARSE SCREEN:

Peak discharge of sewage = 0.924 m3/s

Assume the velocity at average flow is not allowed to exceed 0.8 m/s

The net area screen opening required = 0.9240.8

= 1.16 m2

Clear opening between bars = 30 mm = .03 m

Size of the bars = 75 mm x 10 mm

Assume width of the channel = 1m

The screen bars are placed at 60° to the horizontal.

Velocity through screen at peak flow = 1.6 m/s

Clear area = 1.161.6sin60

= 0.837 m2

No of clear openings = 0.8370.03

=28 Nos

Width of channel = (28 x 30) + (29 x 10)

= 1130 mm = 1.13 m

Provide width of the channel = 1.2 m

Coarse screen channel is designed for the size of

1.2 m X 0.7m (SWD) + 0.5 m (FB)

20

21

22

GRIT CHAMBER

Grit removal basins are the sedimentation basins placed in front of the fine

screen to remove the inorganic particles having specific gravity of 2.65 such as

sand, gravel, grit, egg shells and other non-putrescible materials that may clog

channels or damage pumps due to abrasion and to prevent their accumulation in

sludge digesters. The grit chamber is designed to scour the lighter organic particles

while the heavier grit particles remain settled.

Here the horizontal flow type grit chamber is designed to give a horizontal

straight line flow velocity, which is kept constant over varying discharge.

DESIGN

Peak flow of sewage = 0.924 m3/s

Assume average detention period = 180 s

Aerated volume = 0.924 x 180

= 168 m3

In order to drain the channel periodically for routine cleaning and

maintenance two chambers are used.

Therefore volume of one aerated chamber = 1682 m3

= 84 m3

Assume depth of 3m and Width to depth ratio 2:1

Width of the channel = 2 x 3

= 6 m

Length of the channel = 843 x 6

23

= 4.7 m

Increase the length by about 20% to account for inlet and outlet

Provide length = 4.7 x 1.2 m

= 5.7m

Grit chamber is designed for the size of 5.7m X 6m X 3m

24

25

FINE SCREEN

Fine screens are the structures built between the grit chambers and primary

sedimentation tank in order to remove some amount of suspended solids from

sewage. The fine screens often get clogged need frequent cleaning. The brass metal

is used as it has higher resistant towards rust and corrosion.

Here the disc type fine screen is designed and the wire mesh of the screen is

made up of brass metal. The fine screen is attached with electric motors. The

clogged screen is often cleared by cone brush.

DESIGN

Design flow = 0.924 cumec

At avg. flow design velocity = 0.8 m/s

Area required = 0.9240.8

=1.16 m2

SWD provided = 0.7 m

At peak design velocity = 1.6 m/s

Assuming the screen bars are placed at 40° to the horizontal.

Clear area = .9241.6sin40

= 1.13 m2

Clear opening = 8 mm = 0.008 m

Net clear width of channel = 1.130.008

= 1.41 m

No. of clear openings = 178

No. of bars = 17826

Size of the bars = 50mm x 10 mm

Width of channel = (178 x 8) + (179 x 10)

= 3.2 m

Fine screen is designed for the size of 3.2 m X 0.8 m (SWD) + 0.5 m (FB)

27

28

SKIMMING TANK

Skimming tanks are the tanks removing oils and grease from the

sewage constructed before the sedimentation tanks. Municipal raw sewage contains

oils, fats, waxes, soaps, fatty acids etc. The greasy and oily matter may form

unsightly and odorous scum on the surface of settling tanks or may interfere with

the activated sludge process.

In skimming tank air is blown along with chlorine gas by air diffuser

placed at the bottom of the tank. The rising air tends to coagulate and solidify the

grease and cause it to rise to the top of the tank whereas chlorine destroys the

protective colloidal effect of protein, which holds the grease in emulsified form.

The greasy materials are collected from the top of the tank and the collected are

skimmed of specially designed mechanical equipments.

DESIGN

The surface area required for the tank A = 6.22 X 10-3 X qVr m2

Where q = rate of flow sewage in m3/day

Vr = minimum rising velocity of the oily

material to be removed in m/min

q = 0.924 x 60 x 60 x24

= 79833.6 m3/day

Vr = 0.25 m/min

= 0.25 x 60 x 24

= 360 m/day

A = 6.22 X 10-3 X 79833.6360

A =1.37 m2

29

≈ 1.5 m2

Provide the depth of the skimming tank is 3m

The length breadth ratio is 1.5: 1

Therefore L = 1.5B

L x B = 1.5B2

Therefore B= 1m

L = 1.5 m

Skimming tank is designed for the size of 1.5m X 1m X 3m + 0.5m (FB)

30

31

PRIMARY SEDIMENTATION TANK

Primary sedimentation tank is the settling tank constructed next to

skimming tank to remove the organic solids which are too heavy to be removed i.e.

the particles having lesser size of 0.2 mm and specific gravity of 2.65.

The designed tank is circular type which makes settling by allowing radial

flow. These are fabricated using carbon steel with epoxy lining on the inside and

epoxy coating on the outside. Built on the concept of inclined plate clarification,

these clarifiers use gravity in conjunction with the projected settling area so as to

effect a fairly high percentage of removal of suspended solids as 60 to 65% of the

suspended solids and 30 to 35% of the BOD from the sewage.

DESIGN:

Max. quantity sewage = 26.56 MLD

Surface loading = 40 m3/m2/day

Detention period = 1 hrs

Volume of sewage = 26560 X 124

=1106.7 m3

1110 m3

Provide effective depth = 2.5 m

Surface area =11102.5

= 444 m2

Surface Area the tank = Total flowSurface lloading

32

= 2656040

=664 m2

Use greater of area of these two,

Therefore area surface area of the tank =664 m2

Diameter of the tank = 664 x 4π

=29.07 m

≈29.2 m

Primary sedimentation tank is designed for the dimension of

29.2 m (dia) X 2.5 m (depth) + 0.5 (FB)

33

34

35

ACTIVATED SLUDGE PROCESS

The activated sludge process is an aerobic, biological sewage

treatment system to treat the settled sewage consist a variety of mechanisms and

processes that use dissolved oxygen to promote the growth of biological floc that

substantially removes organic material. The essential units of the process are an

aeration tank, a secondary settling tank, a sludge return line from the secondary

settling tank to the aeration tank and an excess sludge waste line.

CONCEPT:

Atmospheric air is bubbled through primary treated sewage

combined with organisms to develop a biological floc which reduces

the organic content of the sewage. The Mixed Liquor, the combination of raw

sewage and biological mass is formed. In activated sludge plant, once the effluent

from the primary clarifier get sufficient treatment, the excess mixed liquor is

discharged into settling tanks and the treated supernatant is run off to undergo

further treatment. Part of the settled sludge called Return Activated Sludge (R.A.S.)

is returned to the head of the aeration system to re-seed the new sewage entering

the tank. Excess sludge which eventually accumulates beyond R.A.S known Waste

Activated Sludge (W.A.S.) is removed from the treatment process to keep the ratio

of biomass to food supplied (F:M) ratio. W.A.S is further treated by digestion

under anaerobic conditions.

METHOD: CONTACT STABILIZATION METHOD

• Microorganisms consume organics in the contact tank.

• Effluent from primary clarifier flows into the contact tank where it is aerated

and mixed with bacteria.

• Soluble materials pass through bacterial cell walls, while insoluble materials

stick to the outside.

36

FLOW CHART OF CONTACT STABILIZATION ACTIVATED

SLUDGE PROCESS

• Solids settle out later and are wasted from the system or returned to a

stabilization tank.

• Microbes digest organics in the stabilization tank, and are then recycled back

to the contact tank, because they need more food.

• Waste Activated Sludge is removed and sent to further treatment.

PROCESS

37

The activated sludge functions in the above mentioned concept by following

the Contact stabilization method. The effluent from primary clarifier is mixed with

40 to 50% of own volume of activated sludge (R.A.S). Then it is mixed for 4 to 8

hours in the aeration tank by the combined aerator which does compressed air

diffusion and mechanical mixing. The moving organisms oxidize the organic

matter and make it to settle in the secondary clarifier.

The settled sludge known as activated sludge is then recycled to head of

aeration tank and mixed with the new entering sewage. New activated sludge is

produced continuously and W.A.S is disposed along with primary treated sludge

after proper digestion.

The activated sludge plant results 80 to 95% of BOD removal and 90 to 95%

bacteria removal by making the necessary set up such as

(i) Ample supply of oxygen to plant

(ii) Intimate and continuous mixing sewage with activated

sludge.

(iii) Constant rate of return sludge is made to be kept through out

the process.

38

AERATION TANK

Aeration tank is the mixing and diffusing structure in the activated sludge

plant. These are rectangular in shape having the dimensions ranging 3 to 4.5m

deep, 4 to 6m wide and 20 to 200m length. Air is introduced continuously to the

tank.

Combined Aeration type aerators having the diffused air aeration as well as

mechanical aeration together in a single unit are used in the project. The Dorroco

model is designed as it gives higher efficiency and occupies less space. This results

in higher efficiency and lesser detention period and lesser amount of compressed

air.

DESIGN

No. of Aeration tank = 2

Design flow = 26.56 MLD

Average flow of each tank = 265602

= 13280 m3

BOD at inlet = 0.8 x 300

(20 % of BOD removed at Grit chamber)39

Yo = 240 mg/l

BOD at outlet YE = 20 mg/l

BOD Removed in Activated Plant = 240-20

= 220 mg/l

Minimum efficiency required in the activated plant

= 220240

Min. efficiency = 91.7 %

Since the adopted extended aeration process can remove 85-92 %

Hence it is OK

MLSS (Xt) = 3000 mg/l

F/M ratio = 0.4

Volume the tank required V = Q FM x YoXt

= 13280 X 2200.4 X 3000

= 2344.67 m3

≈ 2345 m3

Assume the liquid depth of the tank as 4.5 m

The Width to Depth ratio as 2.2

BD = 2.2

B = 9.9 m

40

≈ 10 m

L = 24354.5 X10

= 54 m

L = 54 m; B = 10 m; d = 4.5 m

Volume provided = 54 x 10 x 4.5

= 2430 m3

(i) CHECK FOR AERATION PERIOD / HRT:

Hydraulic Retention Time (HRT) = t = V X 24Q

= 2430 X 2413280

= 4.39 hrs

Since it lies between 3-6 hrs it is OK.

(ii) CHECK FOR VOLUMETRIC LOADING:

Volumetric loading = Q X YoV

= 13280 X 2402430

= 1171.6 g/m3

=1.171 kg/m3

Since it lies between 1.0 – 1.2 it is OK

(iii) CHECK FOR RETURN SUDGE RATIO:

Return activated sludge = QrQ = Xt(106S.V.I-Xt

Where, S.V.I = Sludge Volume Index

Qr =Sludge Recirculation Rate41

= QrQ = 3000(106115-3000)

= 53%

It lies between 0.5 – 1.0. Design is OK

(iv) CHECK FOR SRT (ӨC):

V x Xt = αy X Q X Yo-YE X Өc1+(KeX Өc)

Where, �y = 0.5 constant for municipal sewage with respect to MLSS

Ke = 0.06 d-1 constant for municipal sewage

ӨC = Solids Retention Time (SRT)

Yo = 240 mg/l

YE = 20 mg/l

V = 2430 m3

Xt = 3000 mg/l

Q = 13280 m3/day

2430 x 3000 = 0.5 X 13280 x(240-20)1+(0.06 X Өc )

1+ 0.06 Өc = 0.2004

Өc = 10.2004

Өc = 7.12 days

It lies between 5-8 days. The deign is OK

Provide the Aeration tank as 54 m X 10 m X 4.5 m + 0.5 m (FB)

42

43

BOD5 applied to each tank = 240 mg/l

Average flow in each tank = 13280 m3/day

BOD5 removed in each tank = 13280 x 0.240

= 3187.2 kg/day

= 133 kg/hr

Oxygen requirement = 1 kg/kg of BOD applied

Peak oxygen demand =125 %

Oxygen transfer capacity of the aeration standard condition

= 1.9 kg/kWh

= 1.41 kg/HP/hr

Oxygen transfer capacity aerators at field conditions

= 0.9 x 1.41

= 1.269 kg/HP/hr

Oxygen to be applied in each tank = 1.0 x 133 x1.25

= 167 kg/hr

HP of aerators required = 1671.269

= 132 HP

Provide 4 Nos. of 40 HP aerators.

SECONDARY SEDIMENTATION TANK

A sedimentation tank constructed next to the aeration tank is the secondary

sedimentation. This tank will be as the primary sedimentation tank with certain 44

modifications as no floating materials are here, provisions for the removal of scum,

floatage are not needed.

The surface area for the secondary sedimentation tank is designed for both

overflow rate basis and solids loading rate basis. The larger value is adopted.

DESIGN

No. of Secondary clarifier = 1

Average flow = 26560 m3/day

Recirculated flow = 53%

= 14070 m3/day

Total inflow = 26560+14070

= 40630 m3/day

Provide hydraulic detention period = 2 hrs

Volume the tank (exclusive of hopper portion)

= 40630 x 224

= 3386.4 m3

Assume liquid depth = 3.5 m

Area = 3386.43.5

= 967.54 m2

Surface loading rate of average flow = 25 m3/m2/day

Surface area provided = 2656025

=1062.4 m2

Using greater area of the two values

45

Therefore surface area = 1062.4 m2

Diameter =1062.4 X 4π

=36.7 m

≈ 37 m

Provide diameter of 37m

(i)CHECK FOR WEIR LOADING:

Average flow = 26560 m3/day

Weir loading = 2656037 X π

= 176.13 m3/day/m

It is lesser than 185 m3/day/m. Hence it is OK

(ii)CHECK FOR SOLIDS LOADING:

Recirculated flow = 14070 m3/day

Average flow = 26560 m3/day

MLSS in the tank = 3000 mg/l

Total solids in flow = (26560+14070) x 3

= 121890 kg/day

Solids loading = 121890967.54

= 125.98 kg/day/m2

It lies between 100-150 kg/m2/day

Hence it is OK

46

Provide secondary sedimentation as 37 m (dias) X 3.5 m (depth) + 0.5 m

(FB)

Hopper slope shall be 1in 12.

STABILIZATION TANK:

Total return flow = 14070 m3/day

= 9.771 m3/min

Detention time = 15 min

Volume of wet well = 9.771 x 15

= 146.6 m3

Provide depth as 3m, width as 5 m

Therefore length is = 9.8 m

Wet well dimension as 9.8m X 5m X 3m + 0.5m (FB)

Dry well dimension as 9.8m X 9.8 m

2 No. of pump house each of 14.07 MLD capacity in the dry well are

provided

SLUDGE DRYING BEDS

Drying of the digested sludge on open beds of land is sludge drying and

such open beds of land are known as sludge drying beds. The digested sludge from

digestion tank contains a lot of water. So it is necessary to dry up or dewater the

digested sludge before it disposed of dumping. It is the quite suitable to dewater in

Vellore due to its hot climate.

The sewage sludge is brought and spread over the top of drying beds to a

depth of 20 to 30 cm, through distribution troughs. A portion of the moisture drains

47

through the bed while most of it gets evaporated to the atmosphere. In hot countries

like India it takes 6 to 12 days to dry. After the period the sludge cakes are

removed with spades and they are used as manure as it contains 2 to 3% of NPK

Sludge drying beds are open beds of land 45 to 60 cm deep, 30 to 45 cm

thick graded layers of gravel or crushed stone varying in size from 15cm at bottom

and 1.25 cm at top. Open jointed under drain pipes of 15 cm diameter are laid

below the gravel layers. Large beds are portioned by concrete walls, and a pipe

header from the digesters with gated openings allows application of sludge

independently to each cell. Seepage collected in the under-drains is returned to the

plant wet well for treatment with the raw wastewater.

DESIGN

Sludge applied to drying bed at the rate of 100kg/MLD

Sludge applied = 300kg/day

Specific gravity = 1.015

Solid content = 2%

Volume of sludge = 3000.02 X 1000 X 1.015

= 14.778m3/day

For Vellore weather condition the beds get dried out about 10 days.

Number of cycle in one year = 36510

= 37 cycles.

Period of each cycle = 10 days

Volume of sludge per cycle = 14.778 X 10

= 147.78 m3

Spreading a layer of 0.3m per cycle,

48

Area of bed required = 147.780.3

= 492.6 m2

≈ 500 m2

Provide 5 nos. of beds,

Area of each bed = 100 m2

5 beds of dimension 12.5m X 8m are designed.

49

SEWAGE DISPOSAL

The disposal of treated effluent into land or water body is sewage disposal.

This can be of two methods,

(i) Dilution – disposal in water bodies.

(ii) Effluent irrigation – disposal on land.

DILUTION:

The disposal of effluent by discharging it into water courses such as streams,

rivers or large body of water such as lake, sea is called dilution.

EFFLUENT IRRIGATION:

When the effluent is evenly spread on the surface of land it is effluent

irrigation. The water of sewage percolates on the ground and the suspended solids

remain at the surface of the ground. The remaining organic suspended solids are

partly acted upon by the bacteria and are partly oxidized by exposure to

atmospheric actions of heat, light and air.

While considering the characteristics of Vellore Corporation it is

preferred that Effluent Irrigation i.e. land disposal for the following reasons.

(i) Vellore Corporation is not a coastal city i.e. sea is out of reach.

Vellore does not have any perennial river makes impossible for

dilution.

(ii) The nearby river stream Pallar has very small amount of dry weather

flow. In summer season it runs dry.

(iii) The Sewage Treatment Plant is designed according to Indian

Standards which produces effluent having lesser hazardous

characteristics than the standards of land disposing.

(iv) It is an alternative source of water for irrigation and it contains the

manure and some amount of NPK compounds.

50

51

Comparison between IS : 3307-1986 and expected effluent’s

characteristics.

The effluent to be disposed in Land Effluent Irrigation method and it is done

by constructing Ridge and Furrow in the disposal land. Here the land is first

ploughed up to 45cm, then leveled and divided into plots and sub-plots. Then each

sub-plot is enclosed by small dykes. Now ridges and furrows are formed in each

sub-plot. The sewage is allowed to flow in furrows, whereas crops are grown on

ridges. After an interval of 8-10 days the sewage can be again applied depending

on the crops requirement and the nature of the soil.

52

Sl.no CharacteristicsTolerance limit as per

IS : 3307-1986Effluent from the plant

1 pH 5.5-9.0 5.5-9.0

2 BOD 100 mg/l ≤ 20 mg/l

3 Suspended solids 200 mg/l ≤ 30 mg/l

4 Oil & Grease 10 mg/l ≤ 5 mg/l

5 Chlorides 600 mg/l ≤ 400 mg/l

6 Sulphate 1000 mg/l ≤ 250 mg/l

SALIENT DETAILS OF PROJECT

Sl.No ATTIBUTE DATA

1 ProjectSewage Treatment Plant for Vellore

Municipal Corporation.

2 Sewerage type Partially Separate Sewerage System

3 Population Census

1951 1,06,024

1961 1,13,742

1971 1,39,082

1981 1,74,247

1991 1,75,061

2001 1,77,413

4 Method of Forecasting Incremental increase method

5 Design Population

Base year-2010 1,89,116

Intermediate 2025 2,10,343

Ultimate year 2040 2,45,920

6 Per Capita Water Supply 135 lpcd

7 Existing Sewerage system Nil

53

CONCLUSION

A successful technical project involves integration of various fields. This is

an attempt to combine several aspects of environmental, biological and chemical

and civil engineering.

Since, in Vellore Municipal Corporation there is no proper treatment plant

for sewage, it is necessary to construct a Sewage Treatment Plant. The plant is

designed perfectly to meet the future expansion for the next 30 years in accordance

with Indian Codal provisions. This project consists the design of the complete

components of a Sewage Treatment Plant from receiving chamber, screening

chamber, grit chamber, skimming tank, sedimentation tank, secondary clarifier,

active sludge tank and sludge drying beds for sewage.

54

55

56

PLANT DETAILS

COMPONENT TYPE NOS DIMENSIONS

Receiving chamber

1 6.2m X 3m X 3m (SWD) + 0.5m (FB)

Coarse screen1 manual

1 mechanical2 1.2m X 0.7m (SWD) + 0.5m (FB)

Grit chamberHorizontal Flow type

2 5.7m X 6m X 3m

Fine screenDisc type,

Mechanical2 3.2 m X 0.8 m (SWD) + 0.5 m (FB)

Skimming tankAir diffuser + Chlorine gas

1 1.5m X 1m X 3m + 0.5m (FB)

Primary clarifierCircular type,

Radial flow 1 29.2m Ø X 2.5m (SWD) + 0.5m (FB)

Aeration tankCombined-

Dorocco type1 54m X 10m X 4.5m + 0.5m (FB)

Secondary clarifier

Circular type,

Radial flow 1 37m Ø X 3.5m (SWD) + 0.5m (FB)

Sludge Drying bed

Sand + Graded

graveled5 12.5m X 8m

57